Re: [RFC PATCH v2 07/19] RISC-V: KVM: Implement KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls

[Anup Patel <anup@brainfault.org>]

On Mon, Aug 5, 2019 at 12:40 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 05/08/19 08:55, Anup Patel wrote:
> > On Fri, Aug 2, 2019 at 2:33 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>
> >> On 02/08/19 09:47, Anup Patel wrote:
> >>> +     if (reg_num == KVM_REG_RISCV_CSR_REG(sip))
> >>> +             kvm_riscv_vcpu_flush_interrupts(vcpu, false);
> >>
> >> Not updating the vsip CSR here can cause an interrupt to be lost, if the
> >> next call to kvm_riscv_vcpu_flush_interrupts finds a zero mask.
> >
> > Thanks for catching this issue. I will address it in v3.
> >
> > If we think more on similar lines then we also need to handle the case
> > where Guest VCPU had pending interrupts and we suddenly stopped it
> > for Guest migration. In this case, we would eventually use SET_ONE_REG
> > ioctl on destination Host which should set vsip_shadow instead of vsip so
> > that we force update HW after resuming Guest VCPU on destination host.
>
> I think it's simpler than that.
>
> vcpu->vsip_shadow is just the current value of CSR_VSIP so that you do
> not need to update it unconditionally on every vmentry.  That is,
> kvm_vcpu_arch_load should do
>
>         csr_write(CSR_VSIP, vcpu->arch.guest_csr.vsip);
>         vcpu->vsip_shadow = vcpu->arch.guest_csr.vsip;
>
> while every other write can go through kvm_riscv_update_vsip.  But
> vsip_shadow is completely disconnected from SET_ONE_REG; SET_ONE_REG can
> just write vcpu->arch.guest_csr.vsip and clear irqs_pending_mask, the
> next entry will write CSR_VSIP and vsip_shadow if needed.
>
> In fact, instead of placing it in kvm_vcpu, vsip_shadow could be a
> percpu variable; on hardware_enable you write 0 to both vsip_shadow and
> CSR_VSIP, and then kvm_arch_vcpu_load does not have to touch CSR_VSIP at
> all (only kvm_riscv_vcpu_flush_interrupts).  I think this makes the
> purpose of vsip_shadow even clearer, so I highly suggest doing that.

Yes, having vsip_shadow as percpu variable makes sense. I will update
accordingly.

>
> >> You could add a new field vcpu->vsip_shadow that is updated every time
> >> CSR_VSIP is written (including kvm_arch_vcpu_load) with a function like
> >>
> >> void kvm_riscv_update_vsip(struct kvm_vcpu *vcpu)
> >> {
> >>         if (vcpu->vsip_shadow != vcpu->arch.guest_csr.vsip) {
> >>                 csr_write(CSR_VSIP, vcpu->arch.guest_csr.vsip);
> >>                 vcpu->vsip_shadow = vcpu->arch.guest_csr.vsip;
> >>         }
> >> }
> >>
> >> And just call this unconditionally from kvm_vcpu_ioctl_run.  The cost is
> >> just a memory load per VS-mode entry, it should hardly be measurable.
> >
> > I think we can do this at start of kvm_riscv_vcpu_flush_interrupts() as well.
>
> Did you mean at the end?  (That is, after modifying
> vcpu->arch.guest_csr.vsip based on mask and val).  With the above switch
> to percpu, the only write of CSR_VSIP and vsip_shadow should be in
> kvm_riscv_vcpu_flush_interrupts, which in turn is only called from
> kvm_vcpu_ioctl_run.

Yes, I meant at the end of kvm_riscv_vcpu_flush_interrupts() but I am
fine having separate kvm_riscv_update_vsip() function as well.

Regards,
Anup

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